X.509数字证书读入程序设计与Java实现

X.509证书结构描述

X.509是密码学中公钥证书的格式标准。在日常生活中，X.509证书已应用在包括TLS/SSL在内的众多Internet协议里，同时它也用在很多非在线应用场景里。X.509证书包含公钥、身份信息（网络主机名、组织或个体的名称等）和签名信息（可以是证书签发机构CA的签名，也可以是自签名）。对于一份经过可信的证书签发机构签名（如CA）或者可以通过其他方式验证的证书，证书的拥有者就可以用证书及相应的私钥来创建安全的通信，对文档进行数字签名。

Certificate
- Version Number
- Serial Number
- Signature Algorithm ID
- Issuer Name
- Validity period
  - Not Before
  - Not After
- Subject name
- Subject Public Key Info
  - Public Key Algorithm
  - Subject Public Key
- Issuer Unique Identifier(optional)
- Subject Unique Identifier(optional)
- Extensions(optional)
  - …
- Certificate Signature Algorithm
- Certificate Signature

下面给出几个例子（使用openssl来查看证书内容）：

Github网站的证书：
Google网站的证书：

数据结构

CertificateFactory：用于指定证书标准类型，在后面用来生成该标准类型的证书。
Certificate：证书的基类。
X509Certificate：从Certificate中继承下来，里面包含了X509的许多特有属性，以及访问这些属性的方法。
RSAPublicKey：RSA加密的公钥。继承PublicKey父类，用于解密输出证书中的公钥算法以及公钥信息。

Java实现

Handler.java:

import java.security.*;
import java.io.*;
import java.util.*;
import java.security.cert.*;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.math.BigInteger;
import java.util.Base64;
import java.security.interfaces.RSAPublicKey;

public class Handler {
  public static void main(String[] args) {
    // Prompt the user to input certificate name
    System.out.print("Please input the certificate: ");
    Scanner input = new Scanner(System.in);
    String certificatePath = input.next();

    Certificate c = null;
    String s;
    try {
      CertificateFactory cf = CertificateFactory.getInstance("X.509");
      // Read Certification
      FileInputStream in = new FileInputStream(certificatePath);
      c = cf.generateCertificate(in);
      in.close();

      s = c.toString();

      // Output as a txt file
      FileOutputStream fout = new FileOutputStream("tmp.txt");
      BufferedWriter out = new BufferedWriter(new OutputStreamWriter(fout));
      out.write(s, 0, s.length());
      out.close();
    }
    catch (Exception ex) {
      ex.printStackTrace();
    }

    // convert certificate to X509
    X509Certificate t = (X509Certificate) c;
    // Print Info
    System.out.println("X.509 Certificate:");
    System.out.println("    Data:");
    System.out.println("        Version: " + t.getVersion());
    System.out.println("        Serial Number: ");
    System.out.println(stringFormat((t.getSerialNumber().toString(16)), 16, 12));
    System.out.println("    Signature Algorithm: " + t.getSigAlgName());
    System.out.println("        Issuer Name: " + t.getIssuerDN());
    System.out.println("        Validity");
    System.out.println("            Not Before: " + t.getNotBefore());
    System.out.println("            Not After : " + t.getNotAfter());
    System.out.println("        Subject: " + t.getSubjectDN());
    // Public Key
    RSAPublicKey publicKey = (RSAPublicKey)t.getPublicKey();
    System.out.println("        Subject Public Key Info: ");
    System.out.println("            Public Key Algorithm: " + publicKey.getAlgorithm());
    System.out.println("                Public-Key: (" + (publicKey.getModulus().toString(16)).length() * 4 + "bit)");
    System.out.println("                Modulus:");
    String st = (publicKey.getModulus()).toString(16);
    st = "00" + st;
    System.out.println(stringFormat(st, 30, 20));
    String exponent = (publicKey.getPublicExponent().toString());
    String exponentHex = (publicKey.getPublicExponent().toString(16));
    System.out.println("                Exponent: " + exponent + " (0x" + exponentHex + ")");

    System.out.println();
    // Signature
    System.out.println("    Signature Algorithm: " + t.getSigAlgName());
    byte[] signature = t.getSignature();
    String sig = (new BigInteger(signature)).toString(16);
    System.out.println(stringFormat(sig, 36, 8));

    byte[] by = t.getExtensionValue("2.5.29.32");
    String str = new String(by);
    System.out.println(str);
  }

  private static String stringFormat(String str, int format, int space) {
    String spaceStr = "";
    for (int i = 0; i < space; i++) {
       spaceStr += " ";
    }
    String result = spaceStr;

    for (int i = 0; i < str.length()-1;) {
      result += str.charAt(i);
      i++;
      result += str.charAt(i);
      i++;
      if (i == str.length()) {
        break;
      }
      else if (i != 0 && i % format == 0) {
        result += ":\n";
        result += spaceStr;
      }
      else if (i % 2 == 0) {
        result += ":";
      }
    }
    return result;
  }
}